Process for preparing precipitated silica

Abstract

A process for preparing precipitated silica, is disclosed. The process includes preparing an aqueous solution of sodium silicate having a pH in a range of 10.6 to 10.8, simultaneously adding to said aqueous solution of sodium silicate, a sodium silicate solution, water and an acid in at least two phases to obtain a reaction mixture, wherein in an initial phase the sodium silicate solution, the water and the acid is added till the sodium ion concentration of the reaction mixture is in range of 0.13 normal to 0.57 normal and in a final phase the sodium silicate solution, the water and the acid is added till the sodium ion concentration of the reaction mixture is in a range of 0.3 to 0.9 normal; acidifying the reaction mixture with the acid to a pH in a range of 3 to 4.5, aging the reaction mixture to obtain precipitated silica and recovering the precipitated silica from the reaction mixture.

We claim: 
     
       1. A process for preparing precipitated silica, the process comprising:
 preparing an aqueous solution of sodium silicate having a pH in a range of 10.6 to 10.8 by adding a sodium silicate solution to water at a temperature in a range of 70 degree Celsius to 160 degree Celsius; 
 simultaneously adding to said aqueous solution of sodium silicate maintained at a reaction temperature in a range of 70 degree Celsius to 160 degree Celsius the sodium silicate solution, water and an acid in at least two phases to obtain a reaction mixture, wherein in an initial phase the sodium silicate solution, the water and the acid is added until the sodium ion concentration of the reaction mixture is in a range of 0.13 normal to 0.57 normal and in a final phase the sodium silicate solution, the water and the acid is added until the sodium ion concentration of the reaction mixture is in a range of 0.3 to 0.9 normal; 
 acidifying the reaction mixture maintained at a reaction temperature in a range of 70 degree Celsius to 160 degree Celsius with the acid to a pH in a range of 3 to 4.5; 
 aging the reaction mixture at a temperature in a range of 70 degree Celsius to 160 degree Celsius for a time period in a range of 5 minutes to 2 hours to obtain precipitated silica; and 
 recovering the precipitated silica from the reaction mixture; wherein the precipitated silica has a CTAB surface area in a range of 150 to 350 m 2 /g and a BET surface area in a range of 190 to 380 m 2 /g, such that the CTAB surface area of precipitated silica is inversely proportional to sodium ion concentration in the initial phase. 
 
     
     
       2. The process as claimed in  claim 1 , wherein between each phase, addition of the sodium silicate solution, the water and the acid is stopped, and the reaction mixture is allowed to age for a time period in a range of 5 minutes to 1 hour at a temperature in a range of 70 degree Celsius to 160 degree Celsius. 
     
     
       3. The process as claimed in  claim 2 , wherein the simultaneous addition of the sodium silicate solution, the water and the acid to the aqueous solution of sodium silicate is carried out in three phases, wherein in the initial phase the sodium silicate solution, the water and the acid is added until the sodium ion concentration of the reaction mixture is in a range of 0.13 normal to 0.57 normal; in an intermediate phase the sodium silicate solution, the water and the acid is added until the sodium ion concentration of the reaction mixture is in a range of 0.23 normal to 0.78 normal and in the final phase the sodium silicate solution, the water and the acid is added until the sodium ion concentration of the reaction mixture is in a range of 0.3 normal to 0.9 normal. 
     
     
       4. The process as claimed in  claim 1 , wherein the sodium silicate solution, the water and the acid are simultaneously added to the aqueous solution of sodium silicate over a time period in a range of 10 minutes to 2 hours. 
     
     
       5. The process as claimed in  claim 1 , wherein the process further comprises adding a surfactant to the aqueous solution of sodium silicate. 
     
     
       6. The process as claimed in  claim 5 , wherein surfactant in a range of 0.5% to 4% wt/wt is added with respect to dry precipitated silica powder. 
     
     
       7. The process as claimed in  claim 1 , wherein the sodium silicate solution has a molarity of sodium silicate in a range of 1.96 moles/litre to 3.87 moles/litre. 
     
     
       8. The process as claimed in  claim 1 , wherein the acid is selected from the group consisting of sulphuric acid, hydrochloric acid, nitric acid, phosphoric acid, and carbonic acid. 
     
     
       9. The process as claimed in  claim 1 , wherein the acid is sulphuric acid having a 50% concentration vol/vol. 
     
     
       10. The process as claimed in  claim 1 , further comprising separating the precipitated silica to obtain a silica cake; washing the silica cake with water to remove unwanted residues followed by homogenising to obtain a silica slurry; and drying the silica slurry. 
     
     
       11. The process as claimed in  claim 10 , further comprising adding to the silica slurry a dispersing agent. 
     
     
       12. The process as claimed in  claim 11 , wherein the dispersing agent is selected from the group consisting of metal salt of saturated and unsaturated fatty esters with long hydrocarbon chain and a mixture of salt of saturated and unsaturated fatty esters with long hydrocarbon chain. 
     
     
       13. The process as claimed in  claim 11 , wherein the dispersing agent comprises calcium oleate, zinc oleate, zinc stearate or mixtures thereof. 
     
     
       14. The process as claimed in  claim 5 , wherein the surfactant is an anionic surfactant selected from the group consisting of sodium dioctyl sulfosuccinate, sodium oleate, and sodium dodecyl sulphate. 
     
     
       15. The process as claimed in  claim 5 , wherein the surfactant is a cationic surfactant. 
     
     
       16. The process as claimed in  claim 15 , wherein the cationic surfactant is cetyl tri methyl ammonium bromide.